1. (Field of the Invention)
The present invention relates to a semiconductor switching circuit used in the millimeter-wave band.
2. (Description of Related Art)
Field effect transistors (FET) are typically used as a switching element for switching between transmitting and receiving signals in a communication, receiving, or transmission module used in microwave and millimeter-wave communications and radar systems.
FIG. 17A is a front view of a FET 600 used as a single-pole single-throw (SPST) switch in a typical semiconductor switch, and FIG. 17B is a sectional view taken along the line XVIIB-XVIIB' in FIG. 17A. Drain interconnect 601 and drain electrode 602 are connected together by means of a conductive air bridge 617 bridging source electrode 605 and gate electrode 612. Drain electrode 602 and drain electrode 603 are connected together by a conductive air bridge 618 bridging source electrode 606 and gate electrodes 613 and 614. Drain electrode 603 and drain interconnect 604 are connected together by a conductive air bridge 619 bridging source electrode 607 and gate electrodes 615. Source electrodes 605, 606, and 607 are connected to via hole 609 by way of a generally comb-shaped source interconnect 608. Gate electrodes 612, 613, 614, and 615 are interleaved with a gate current supply interconnect 616 between the above-noted source and drain electrodes. The drain interconnect 601 is connected to a transmission line 610 forming a part of an MMIC (Microwave and Millimeter-wave Integrated Circuit). Drain electrode path 604 is similarly connected to a transmission line 611 also forming another part of the MMIC.
FIG. 18 shows an equivalent circuit of the FET 600. Inductances 623 and 624 disposed in front and rear stages of the FET 600, respectively, have an inductance component L peculiar to the FET 600 as shown in FIG. 17A, and inductance 625 is an inductance component Ls of the via hole 609 shown on the left side of source electrodes 605, 606, and 607 in FIG. 17A.
Switching is accomplished by controlling the voltage (which is hereinafter referred to as "gate voltage Vg") applied to the gate electrodes, that is, to gate current supply interconnect 616, of FET 600. More specifically, FET 600 is on when gate voltage Vg is set to a level lower than or equal to a specific threshold value, such as when the gate voltage Vg is set to approximately 0 V, to thereby connect transmission line 610 to ground conductor 622. As a result, there is no signal flow to transmission line 611. When the gate voltage Vg exceeds the above-noted threshold value, FET 600 is off, signal flow from transmission line 610 to ground conductor 622 is interrupted, and signals thus flow from transmission line 610 to transmission line 611.
FIG. 19 is an equivalent circuit of FET 600 in the ON state. Resistor 626 is an ON resistance R.sub.on. Impedance Z.sub.on of the FET observed at node B is expressed by the following equation: EQU Z.sub.on =R.sub.on +j2 .pi.f(2L+Ls).
As will be known from this equation, impedance Z.sub.on increases as the frequency f of the RF signal input increases. When impedance Z.sub.on reaches a particular high level, resistance division allows part of the signal that should flow from transmission line 610 to ground conductor 622 to leak to transmission line 611, and switching characteristics deteriorate, that is, signal loss increases and isolation deteriorates.
FIG. 20 is an equivalent circuit of FET 600 in the OFF state. Capacitance 627 is an OFF capacitance C.sub.off. Impedance Z.sub.off of the FET observed at node B is expressed by the following equation: EQU Z.sub.off =-j/2 .pi.fC.sub.off +j2 .pi.f(2L+Ls)=-j[1-4 .pi..sup.2 f.sup.2 C.sub.off /(2L+Ls)]/(2 .pi.fCoff).
As will be known from this equation, impedance Z.sub.off decreases as the frequency f of the RF signal increases. When impedance Z.sub.off reaches a particular low level, resistance division allows part of the signal that should flow from transmission line 610 to transmission line 611 to leak to ground conductor 622, and switching characteristics again deteriorate, that is, signal loss increases and isolation deteriorates.
FIG. 21 is a Smith chart showing impedance Z.sub.on and impedance Z.sub.off, indicated by the black dots in the figure, at node B in FIG. 19 and FIG. 20 when an RF signal of frequency f=75 GHz is passed. As noted above, impedance Z.sub.on when in the ON state and impedance Z.sub.off in the OFF state are proportional to the frequency f of the RF signal. To improve switching characteristics with high frequency RF signals, particularly in the millimeter-wave band, inductances 623, 624, and 625, or more specifically the inductance L of the FET design and the inductance Ls of the via hole, must be suppressed to low levels.